U.S. Pat. No. 7,573,362 discloses a high-current, multiple air gap, conduction-cooled, stacked lamination inductor. The magnetic core section of this known inductor includes substantially rectangular profiled magnetic laminations arranged in a stack.
Generally, in order to reduce the size of power electronics devices, converters are designed to use working frequencies that, for small power converters up to 10 V, have risen into the MHz range. There continues to be research on designing middle-power converters, up to 200V, and high-power converters, up to 500V, that can operate at frequencies in the range of 300 kHz up to 1 MHz.
In such converters, the inductor presents an important part regarding the losses and the size. Particularly, the inductor's size should be minimal. If possible, the inductor shape should be square and the inductor should have the lowest possible AC/DC resistance ratio at the desired working frequency. In existing inductors used at high frequencies, the skin effect, proximity effect, and fringing effect all contribute to comparatively high losses and correspondingly required large size.
In order to obtain the smallest possible inductor with a low DC resistance, the most known switching-converter inductors are wound with a circular or square wire on different shape ferrite cores with one or two air gaps. Better results are reached with inductors having their winding enclosed in a powdered material that, due to low permeability, replaces the air gap.
Relatively good results are achieved by the prior art inductor shown in FIG. 14, where TC denotes a toroidal ferrite core with an air gap AG and having strand wire SW wound around the core TC. These prior art inductors show a favorable AC/DC current resistance ratio. However, their field radiation is high, they are physically quite large, and their shape is inconvenient for fixing on a circuit board.
High-frequency current in circular or square-shape free wires is conducted only on the wire's surface. As a result of this phenomenon, which is known as the “skin effect,” known inductors that have been wound with such wires have a resistance that varies dramatically with increasing frequency. The resulting high-frequency losses make these known inductors only useful for low alternating current frequencies.
The air gap also contributes to an increase the high-frequency losses. The magnetic flux exits the core in the area of the air gap and enters the winding, causing heating of the winding. Even the replacement of a single air gap by plural air gaps does not reduce the effect of this heating phenomenon very much at high frequencies. Although the effect can be completely eliminated by using a composite ferrite material as a core material, the permeability of a corresponding inductor depends very much on the magnetic density. Moreover, the composite ferrite material has a much lower saturation level than the sintered ferrite material. This means that the inductivity of such composite ferrite material inductors varies drastically with current changes.